1. Field of the Disclosure
The present disclosure relates to treated inorganic powders, more particularly treated titanium dioxide, having an improved loose bulk density; a process for their preparation; and their use in polymer compositions.
2. Description of the Related Art
High molecular weight polymers, for example, hydrocarbon polymers and polyamides, are melt extruded into shaped structures such as tubing, pipe, wire coating or film by well-known procedures wherein a rotating screw pushes a viscous polymer melt through an extruder barrel into a die in which the polymer is shaped to the desired form, and is then subsequently cooled and solidified into a product, that is, the extrudate, having the general shape of the die. In film blowing processes, as an extruded plastic tube emerges from the die the tube is continuously inflated by air, cooled, collapsed by rolls and wound up on subsequent rolls.
Inorganic powders can be added to the polymers for various reasons. In particular, titanium dioxide pigments, are added to polymers for imparting whiteness and/or opacity to the finished article. To deliver other properties to the molded part or film, additional additives are usually incorporated into the processing step.
In a typical method for combining inorganic powders and polymers the powder and polymer are dropped through a feed tube into the feed barrel near the starting end of an extruder or into a “side stuffer” part-way along the extruder's length. The combined powders and polymers are compounded and extruded.
In another typical method for combining inorganic powders and polymers, the inorganic powder can be dropped with the polymer into the cavity of a rotational blender such as a Banbury.
For ease of processing, it is desirable for bulk dry powders used as ingredients in industrial operations, and in polymer compositions in particular, to flow freely. One measure of the free-flow property of a powder is bulk flow. Bulk flow is a general term that describes the flowability of a powder in storage and handling systems. One measure of bulk flow is the Rat Hole Index (RHI) as measured by a Johanson Hang-up Indicizer which is described herein below.
In the field of compounding inorganic powders with polymers, the powders are typically received by the polymer compounder in packages containing from 20 kg to more than 20 tons. Smaller package volumes (typically up to 1 metric ton) are added to the process via small hoppers or “day bins” that can store sufficient powder for production periods ranging between a few minutes and a full day. Larger packages (from 1 to 20-25 tons) are typically unloaded into silos that may contain sufficient powder for many days of production. This unloading and powder transfer may be performed by pneumatic or mechanical conveyors or simple gravity chutes. In all of these cases, it is desirable for the powder to have sufficient flowability to discharge reliably from the hopper or silo without blockages and, preferably, without the need for extraordinary flow promotion efforts. Many inorganic powders, especially titanium dioxide, are known for having poor flow properties, as compared to free-flowing materials such as dry sand or plastic pellets. Handling systems for the subject materials are frequently operating near their limits; consequently, powder treatments that reduce the flowability of the dry bulk powder increase the compounder's handling costs. In particular, arching (bridging) and a related form of flow obstruction known as ratholing may occur with powders that have poor flow properties, making it difficult if not impossible to empty the silo or hopper without extensive human intervention which may require process shutdown. Powder flow properties also affect the filling of the screw flights of metering screws and feed screws of extruders. Poor flowing powders tend to not fill the flights in a consistent way, and also not fill them as completely as a more flowable powder would. This reduces both the uniformity (accuracy) of a screw transport system and also its delivery capacity.
The need for improved productivity through higher output of compounded polymer is a constant issue with both blending and compounding methods. In each method, the production rate can be limited by the physical volume of the apparatus used to introduce the mixture of inorganic powder and polymer into the process. Since the feeding devices are limited by volume, not mass, increasing the loose bulk density of the inorganic powder (the density of the powder in loose form) is one possible way to increase the mass that can be processed. Both methods require that the powder flow readily into the reaction chamber. In the case of the extruder, the rate at which the compounding can occur may be limited by the transport capacity (volume per revolution) of the rotating screw. If the powder has a higher loose bulk density, more mass can be transported per revolution of the screw, resulting in higher output. Similarly the total output of a rotational blender may be limited by the volume occupied by the individual components prior to blending. To improve the productivity of these blenders, it is desirable to decrease the amount of space a given mass powder component takes up. Therefore, if the loose bulk density of the inorganic powder is increased, it will take up less volume and increase the overall output of the blender.
While higher values of loose bulk density, in themselves, are beneficial for bulk flow, some titanium dioxide pigments with high loose bulk density are also highly compressible. The compressibility is indicative of interparticle packing which results in greater cohesive strength and poor bulk flow.
In addition, powder treating techniques, in particular treatment with organosilicon compounds to improve performance properties such as lacing resistance in a polyolefin matrix, can have a detrimental impact on the bulk flow properties of the powder.
A treatment for inorganic powders which improves the loose bulk density of the powder without a significant impact on bulk flow has been discovered.